The present invention relates to a sputtering target which allows direct current (DC) sputtering upon forming a film via sputtering, has minimal arcing during sputtering, is capable of reducing particles (dust) and nodules resulting therefrom, has high density and small variation in quality, and is capable of improving the productivity, as well as to the manufacturing method thereof. The present invention also relates to a thin film for an optical information recording medium (particularly used as a protective film) obtained by using the said target as well as to the manufacturing method thereof.
In recent years, technology of high density optical recording disks such as high density optical information recording mediums capable of rewriting without requiring a magnetic head has been developed, and these disks are rapidly attracting attention. This optical disc can be classified into the three categories of ROM (read-only), R (write-once) and RW (rewritable). Particularly, the phase change method employed in the RW type discs is attracting attention. The recording principle employing this phase change optical disk is briefly explained below.
This phase change optical disc performs the recording/reproduction of information by heating and increasing the temperature of a recording thin film on a substrate by irradiating a laser beam thereto, and generating a crystallographic phase change (amorphouscrystal) in the structure of such recording thin film. More specifically, the reproduction of information is conducted by detecting the change in the reflectivity caused by the change in the optical constant of the phase.
The aforementioned phase change is performed with the irradiation of a laser beam narrowed down to a diameter of approximately several hundred nm to several μm. Here, for example, when a 1 μm laser beam passes through at a linear velocity of 10 m/s, light is irradiated to a certain point on the optical disc for 100 ns, and it is necessary to perform the aforementioned phase change and detect the reflectivity within such time frame.
Moreover, in order to realize the foregoing crystallographic phase change; that is, the phase change of the amorphous and crystal, not only will the recording layer be subject to repeated heating and rapid cooling, the peripheral dielectric protective layer and the reflective film of aluminum alloy will also be repeatedly subject thereto.
In light of the above, a phase change optical disc has a four-layer structure wherein, for instance, both sides of a Ge—Sb—Te recording thin film layer or the like are sandwiched with a ZnS—SiO2 high-melting point dielectric or the like, and an aluminum alloy reflective layer is additionally provided thereto.
Among the above, in addition to being demanded of an optical function capable of increasing the absorption of a laser beam at the amorphous portion and crystal portion of the recording layer, and which has a large reflectivity difference, the reflective layer and protective layer are also demanded of a function for preventing the deformation caused by the moisture resistance or heat of the recording thin film as well as a function for controlling the thermal conditions upon recording (c.f. “Kogaku” magazine, volume 26, no. 1, pages 9 to 15).
As described above, the protective layer of a high-melting point dielectric must be durable against repeated thermal stress caused by the heating and cooling, must not allow such thermal effect to influence the reflective film or other areas, and it is also required to be thin, of low reflectivity, and possess strength to prevent deterioration. From this perspective, the dielectric protective layer plays an important role.
The dielectric protective layer described above is usually formed with the sputtering method. This sputtering method makes a positive electrode substrate and a negative electrode target face each other, and generates an electric field by applying a high voltage between the substrates thereof and the targets under an inert gas atmosphere. The sputtering method employs a fundamental principle where the inert gas are ionized, plasma which consists of electrons and the positive ion is formed, the positive ion in this plasma extrudes the atoms structuring the target by colliding with the target (negative electrode) surface, and the extruded atoms adhere to the opposing substrate surface, wherein the film is formed thereby.
Conventionally, ZnS—SiO2 has been widely used as the protective layer of a rewritable optical recording medium due to its superior characteristics regarding optical characteristics, heat characteristics, and adhesiveness with the recording layer. Nevertheless, a rewritable DVD is demanded of increased number of rewritings in addition to the realization of shorter wavelengths of the laser wavelength, realization of a large capacity and high speed recording are also strongly demanded, and the characteristics of conventional ZnS—SiO2 are becoming insufficient.
As one reason that the number of times the optical information recording medium can be rewritten will deteriorate, there is a problem in that the sulfur constituent from the ZnS—SiO2 will be diffused to the recording layer material disposed between ZnS—SiO2. Also, pure Ag or Ag alloy having high reflectivity and high thermal conductance characteristics for realizing large capacity and high speed recording is being used as the reflective layer material.
This reflective layer is also disposed adjacent to the ZnS—SiO2 protective layer material, and, due to the diffusion of the sulfur constituent from ZnS—SiO2, the pure Ag or Ag alloy reflective layer material would become corroded and deteriorate, and caused the deterioration in the characteristics of the reflectivity and so on of the optical information recording medium.
Although an intermediate layer having nitride or carbide as its principal component is provided between the reflective layer and protective layer, and between the recording layer and protective layer in order to prevent the diffusion of the sulfur constituent, there are problems in that the throughput will deteriorate and costs will increase as a result of the increased number of layers.
In order to overcome these problems, a material having similar characteristics as with the ZnS—SiO2 protective layer material that does not contain ZnS is being sought. Further, since SiO2 often causes the deterioration in the deposition rate and abnormal electrical discharge, it is desirable to avoid adding the same.
In light of the above, the use of a material having as its principal component a ZnO base homologous compound (c.f. technical journal “Solid Physics” C. Li et al., Vol. 35, No. 1, 2000, page 23 to 32 “Microscopic Observation of Modulated Structure of Homologous Compound RMO3(ZnO)m (R=In, Fe; M=In, Fe, Ga, Al; m=natural number)”) that does not contain ZnS and SiO2 has been considered.
Since a ZnO base homologous compound has a complex layer structure, it is characterized in that it is capable of stably retaining the amorphous nature during deposition, and in this respect, this compound yields the same effect as the addition of SiO2. Further, this is transparent in the used wavelength range, and the refractive index is also similar to ZnS—SiO2.
As described above, by reducing or eliminating the influence of the sulfur constituent by using a material having oxide as its principal component as a substitute for a ZnS—SiO2 protective layer material, this has been expected to improve the characteristics of the optical information recording medium, and improve the productivity thereof.
Generally speaking, as examples of using a material having a homologous compound as its principal component as a transparent conductive material, for instance, there is a method of forming a zinc-indium oxide target via laser abrasion (c.f. Japanese Patent Laid-Open Publication No. 2000-26119), an example of a transparent conductive film containing amorphous nature oxide and having favorable conductivity and in particular favorable blue light permeability (c.f. Japanese Patent Laid-Open Publication 2000-44236), and an example of a moisture resistance film forming target having In and Zn as its principal components, which is In2O3(ZnO2)m (m=2 to 20), and the atomic ratio of In and Zn(In/(In+Zn)) is 0.2 to 0.85 (c.f. Japanese Patent No. 2695605).
Nevertheless, it could not be said that the material forming the foregoing transparent conductive film was sufficient as a thin film for an optical information recording medium (in particular for use as a protective film). Meanwhile, with the ZnO based homologous compound, there is a problem in that it is difficult to increase the bulk density, and only a low density sintered body target can be obtained.
With this kind of low density target, there are problems in that arcing easily occurs during the formation of the film via sputtering, particles (dust) and nodules will generate during sputtering as a result of such arcing, and, not only will the uniformity and quality of deposition deteriorate, the productivity will also deteriorate.